1. Field of the Invention
The present invention relates to a low noise amplifier, and more particularly, to a low-noise and high-gain low noise amplifier.
2. Description of the Prior Art
With the widespread usage of cellular phones, mobile communication has become an integral part of daily life. Many design companies endeavor to improve every circuit block of the communication system. Low noise amplifiers (LNAs) belong to the receiver part of a communication system, with the function to enlarge received signals and to suppress the receiver's noise.
Commonly, LNA structure is based on a single-input-to-single-output design. In this structure the input end of the mixer that follows the LNA has to be single-ended as well. This design has limited ability to reduce the common mode noise of the mixer and the signal leaked from the oscillator to the mixer. Applying a differential output structure to the LNA can solve the problem. The most simple and common way to achieve a LNA with a differential output is by designing a differential-input-to-differential-output structure. This structure requires an extra transformer to convert a single-ended signal received at the antenna to a differential signal at the output end. This transformer not only adds extra cost to the capital, but its power loss also increases the NF (Noise Figure) of the entire receiver and encumbers system performance. Therefore, the preferred design for a receiver is a LNA with a single-input-to-differential-output structure.
Please refer to FIG. 1. FIG. 1 is a diagram of a prior art LNA 10 based on a single-input-to-differential-output design. The LNA 10 includes a transformer 12 and a differential amplifier 14. The transformer 12 is a passive single-input-to-differential-output transformer formed by the metal coils on the integrated circuit. The differential amplifier 14 comprises a differential pair of transistors M2 and M3, and an output impedance ZL for matching the output impedance seen into RFout. The transformer 12 is coupled to the differential amplifier 14 to amplify the high frequency signal entering at the input end RFin. The metal coils of the transformer 12 occupy large areas of the circuit layout and add to manufacturing costs.
Please refer to FIG. 2. FIG. 2 is a diagram of a prior art LNA 20 based on a single-input-to-differential-output design. The LNA 20 includes a first-end input impedance 21, a second-end input impedance 22 and a differential amplifier 24. The differential amplifier 24 comprises a differential pair of transistors M2 and M3, and an output matching impedance ZL. The gate of one of the differential pair transistors is coupled to ground through the second-end input impedance 22. The gate of the other transistor is coupled to the input end RFin through the first-end input impedance 21. The prior art LNA structure 20 is advantageous over the prior art LNA structure 10 in that it removes metal coils serving as transformers, saves more space for other circuitry and reduces manufacturing costs. When a differential amplifier is operated at high frequencies, however, the current source Is used to bias the differential pair cannot be viewed as an ideal high impedance current source. Thus, when designing for the noise and gain for the prior art LNA 20, the transistor M2 cannot be treated as a common-source structure. Therefore the prior art LNA 20 requires complicated impedance matching designed at both input end and output end.
If a metal coil transformer is used to achieve a single-input-to-differential-output LNA structure, large areas on the circuit will be occupied, raising manufacturing costs. On the other hand, if a single-input-to-differential-output LNA structure is achieved by grounding one input end of the LNA, as demonstrated in prior art LNA 20, the high frequency impact of the current source on the differential transistors has to be taken into consideration. This high frequency characteristic of a non-ideal current source increases the complexity when designing the noise and gain for the LNA.